Effect of dissociation equilibria on ion‐exchange processes of weak electrolytes

Abstract

ion-exchange processes involving weak electrolytes are influenced strongly by the pH via the occurrence of association/dissociation equilibria. This influence was investigated quantitatively through binary displacement experiments with different electrolytes to gain better insight in the role of the pH and to test a jived-bed ion-exchange model used for the description of multicomponent processes. The two major factors determining the ion-exchange behavior of weak electrolytes are the actual exchange of counterions and the sorption of neutral species. The sorption of neutral species resulted in concentration fluctuations superimposed on the normal ion-exchange profiles, whereas pH fluctuations inherent to ion-exchange processes in general also influenced concentration through modified ionic ji-actions. The relative importance of these contributions is determined by the magnitude of parameters such as resin capacity, distribution coefficients of neutral species, and selectivities of ions, as well as by operating conditions such as concentrations of all species.

Behavior of the Inadvertent pH Transient Formed by a Salt Gradient in the Ion-Exchange Chromatography of Proteins

“…More recently, Nicolas-Simonnot et al (3) performed full numerical simulations of the governing partial differential equations that account for both mass-transfer effects and the adsorption of neutral species for the same cases considered by Helfferich and Bennett and obtained results that largely agreed with all of the experimental data. Related numerical simulations that account for the behavior of pH gradients in chromatographic columns are discussed by Jansen et al (4,5). Theoretical and experimental studies of the ion exchange of weak electrolytes pertinent to the process of chromatofocusing, which involve a greater number of buffering species than considered by either Helfferich and Bennett, NicolasSimonnot et al, or Jansen et al have been performed by Frey and co-workers (6)(7)(8)(9).…”

Buffer salt effect on pH in the interior of an anion exchange resin

“…The determination of this parameter was not repeatable, due to low H + concentrations at moderate pH, resulting in large variations in the internal pH of the resin phase [8]. In subsequent work, this model was applied to whole column experiments to predict the effluent composition [9,10]. Uptake of undissociated weak electrolyte species by the resin is suspected to contribute to the appearance of delayed pH gradients in ion exchange columns [9].…”

“…The complete exclusion of co-ions is rarely achieved in reality, and this assumption leads to inaccuracies in the estimation of species concentrations in the resin phase [4]. There have been efforts to relax this assumption and develop more broadly applicable models [8][9][10][11][12][13], however, these models still rely on adjustable parameters that must be fit to experimental data. Jansen et al [8] visualize the resin as a homogeneous phase in which the active sites are randomly distributed and uses a Donnan approach to estimate pH of the resin phase.…”

“…The stoichiometric relationship may [1][2][3]6] or may not [7][8][9][10][11] be incorporated into the model to account for interactions between mobile species and resin fixed-charge sites. When the stoichiometric relationship is not included, the phase equilibrium is usually seen to be described by means of an equilibrium constant [7] or selectivity factor [9][10][11]. Many of these models assume that coions are essentially excluded from the resin phase due to Donnan equilibrium forces [4][5][6][7].…”

“…The resin can be visualized as a homogeneous phase with the charges distributed evenly throughout [1][2][3][4][5][6][7][8][9][10][11]. The stoichiometric relationship may [1][2][3]6] or may not [7][8][9][10][11] be incorporated into the model to account for interactions between mobile species and resin fixed-charge sites.…”